1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, an electrophotographic apparatus and a process cartridge.
2. Description of the Related Art
Electrophotographic apparatuses are used in, for example, copiers and laser beam printers, since they have high process speed and printing quality.
For photoconductors used in electrophotographic apparatuses, development has actively been made on organic photoconductors (OPCs) using organic photoconductive materials, and OPCs have been used gradually widely.
Also, the structure of photoconductors has been changed from a single-layered structure where a charge-transfer complex or a charge generating material is dispersed in a binder resin, to a functionally-separated structure where a charge generation layer and a charge transport layer are responsible for respective functions, and their performance has been improving.
In addition, an intermediate layer has been provided as a layer constituting a photoconductor in order to, for example, improve adhesiveness to a photoconductive layer, coating property of a photoconductive layer and chargeability, to prevent unwanted charges from being injected from a substrate, and to cover defects on a substrate.
For example, functionally-separated photoconductors that are mainly used at present have a structure where a charge generation layer and a charge transport layer are provided on an intermediate layer which has previously been provided on an electroconductive substrate.
Conventionally known resins used for intermediate layers include water-soluble resins such as polyvinyl alcohols and casein; alcohol-soluble resins such as Nylon resins; polyurethanes, melamin resins, phenol resins, alkyd resins, epoxy resins and siloxane resins.
Moreover, curing such resins with heating to form three-dimensional network structures has been attempted to increase their resistance to solvents.
Among them, such resins as melamin resins, alkyd/melamine resins, acryl/melamine resins, phenol resins and polyamide copolymers are used since these are superior in providing stable coating liquids.
There has also been proposed a photoconductor having an intermediate layer the resin of which contains a metal oxide serving as an inorganic pigment dispersed therein.
When specular reflection occurs on a surface of an intermediate layer in writing by coherent light, specularly reflected light interfaces together to make an image involve unevenness in density in moiré pattern.
However, incorporation of a metal oxide into an intermediate layer as a white pigment can prevent specular reflection on the surface of the intermediate layer to suppress generation of moiré.
Also, when a surface of a photoconductor is charged in a charging step, opposite charges are induced on the substrate side. In this case, when the electrical resistance of an intermediate layer is too low, the intermediate layer cannot block injection of charges from the substrate into the photoconductive layer. As a result, portions where charges are injected from the substrate into the photoconductive layer are not sufficiently charged to cause image defects such as black spots.
Meanwhile, when the electrical resistance of an intermediate layer is too high, the intermediate layer blocks positive and negative charges generated in a charge generation layer that are to be transferred to the substrate side upon exposure to light, leading to increase in residual potential on the surface of the intermediate layer. In one measure to suppress injection of charges from the substrate to the photoconductive layer and increase in residual potential, a metal oxide which is an electroconductor is added to a relatively electrically insulating resin, and the ratio between the metal oxide and the resin and the thickness of an intermediate layer are controlled to adjust the electrical resistance of the intermediate layer. This measure can overcome such disadvantages to some extent but when it is taken alone, improvements made on the resultant intermediate layer are limited.
Also, when a photoconductor is repeatedly used, charges trapped in its intermediate layer in a charging step causes delayed charging, in which the surface potential of the photoconductor does not increase right after charges are given to the photoconductor but normal charging starts after a certain amount of charges flows into the photoconductor.
Even when a photoconductor involving delayed charging is subjected to a charging process under such conditions as to provide a normal photoconductor with a sufficient charge potential, the surface potential of the photoconductor cannot reach a desired level before imagewise light exposure, so that unevenness in density of images disadvantageously occurs.
When a metal oxide is incorporated into the intermediate layer, the metal oxide is appropriately selected or subjected to surface treatments or various additives are added to the intermediate layer, in order to attain stable electrical characteristics and prevent formation of abnormal images such as black spots. For example, there are proposed methods for suppressing increase in residual potential of a photoconductor or formation of images with black spots or both of them, by incorporating, into an intermediate layer, a metal oxide whose surface has been treated with an organic silicon compound (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-57862, 2003-66636 and 2002-196522).
Another proposed method forms an intermediate layer containing a coupling agent having an unsaturated bond, a metal oxide and a binding agent, suppressing increase in residual potential of a photoconductor and improve storage stability of a coating liquid (see, for example, JP-A No. 11-15184).
Still another proposed method forms an intermediate layer containing polyol-coated titanium oxide particles and a binder resin, improving electrical characteristics and imaging characteristics of an electrophotographic photoconductor in a wide range of environments from high-temperature, high-humidity environments to low-temperature, low-humidity environments (see, for example, JP-A No. 10-228125).
Yet another proposed method incorporates zirconium oxide into an intermediate layer in an amount of 20% by mass or more, improving environmental stability and reducing image defects (see, for example, JP-A No. 11-202518).
Even another proposed method incorporates a white metal oxide or metal fluoride into an intermediate layer and incorporates a polyalkylene glycol into a charge generation layer or the intermediate layer, reducing residual potential and avoiding delayed charging due to fatigue (see, for example, JP-A No. 05-165241).
These methods can prevent increase in residual potential and formation of images with black spots but cause delayed charging due to repetitive use of the photoconductors. As such, there have been no methods that do not cause delayed charging and do reduce increase in residual potential and formation of images with black spots.
Therefore, at present, demand has arisen for an electrophotographic photoconductor that changes in characteristics to a lesser extent even after repetitive use and involves increase in residual potential and formation of images with black spots to a lesser extent.